Use of nitrogen oxides to inhibit coke formation in hydrocarbon thermal cracking processes



March 7, 1967 J. A. SCHULTZ USE OF NITROGEN OXIDES TO INHIBIT COKEFORMATION IN HYDROCARBON THERMAL CRACKING PROCESSES Filed May 18, 1966Ni/r'ogen Hydro carbon AH'r and 01/049; 680 0/900 Prehea/er flea/ea90666 Hea/ Cracker C racked gases 60, 602, N2, r

(Inca/7049 7360 0566 L/gu/fi/eo cracked gases INVENTOR. John 4 Salad/ BYHTTORNEYS United States Patent 3,308,056 USE OF NITROGEN OXIDES TOINHIBIT COKE FORMATION IN HYDROCARBON THERMAL CRACKING PROCESSES John A.Schultz, Bay City, Mich., assignor to The Dow Chemical Company, Midland,Mich., a corporation of Delaware Filed May 18, 1966, Ser. No. 557,335 3Claims. (Cl. 208128) This is a continuation-in-part of my copendingapplication Serial No. 291,499, filed June 28, 1963, now abandoned.

This invention relates to an improvement in conventional thermalhydrocarbon cracking techniques and, more particularly, to the additionof an oxide of nitrogen to the hydrocarbon feed to minimize carbonformation in the cracking process.

Coke formation as a result of the thermal cracking of hydrocarbons isundesirable and has long been a problem in that the cokeformed tends toplug the reactor tubes. Up until now, this necessitated the stopping ofthe cracking process periodically and removing the carbon by oxidizingit to carbon monoxide and carbon dioxide with air and steam.

Thus, it is a principal object of the present invention to provide amethod for minimizing coke formation in thermal cracking processesthereby minimizing reactor plugging and down time for decoking and thusincreasing the efficiency of the cracking operation.

A further object of the present invention is to provide a method forminimizing coke formation in thermal cracking processes, said methodproducing gaseous products which are readily separable from aromatichydrocarbon products.

These and other objects and advantages will become more apparent afterreading the detailed description of the method of this inventiondisclosed hereinafter, reference being made to the accompanying drawingshowing a flow diagram of the method of the invention.

In accordance with the method of the present invention, a nitrogen oxideproduct is added to the input hydrocarbon feed to a thermal hydrocarboncracking process and both said nitrogen oxide product and said feed aresubmitted to cracking. The nitrogen oxide product oxidizes the carbon orcarbon-forming species produced by cracking of the hydrocarbon feedthereby forming byproducts in the cracking procedure. The so-producedbyproducts are gaseous products such as carbon monoxide, carbon dioxide,nitrogen and sometimes, primary amines. These gaseous products areeasily separated from the desirable products and removed therefrom.

This improvement in cracking, that is, the addition of a nitrogen oxideproduct to the hydrocarbon feed, may be used in connection with anyconventional thermal cracking operation in which hydrocarbon issubjected to pyrolysis. In the process of this invention, as in thestandard thermal cracking process, it is necessary that substantially noair or oxygen be present in the reaction zone during the crackingprocess.

In general, any of the oxides of nitrogen may be used in the method ofthis invention including nitrous oxide (N 0), nitric oxide (NO),nitrogen dioxide (N0 nitrogen trioxide (N0 dinitrogen tetroxide (N 0dinitrogen trioxide (N 0 and nitrogen pent-oxide (N 0 3,308,056 PatentedMar. 7, 1967 ICC The range of mole ratios of the nitrogen oxide productto the hydrocarbon feed needed for eflicient operation of the method ofthis invention is determined by the nature of the hydrocarbon feed, thenature of the nitrogen oxide product and the amount of coke formedduring the cracking process in question which must be burned out inorder to continue the operation wherein no nitrogen oxide product isadded to the feed stream. The quantity of coke burned out may bedetermined by analysis of the quantity of air and/or steam needed inaccomplishing the burn-out. For example, where propylene is to becracked, a range of from 0.1 to 10 mole percent of a nitric oxide may beused, with a preferred range being from 0.5 to 3 mole percent.

The method of this invention may be carried out at any temperature atwhich thermal cracking is accomplished, such as from about 600 to 1050C. In cracking propylene, for example, a temperature in the range offrom 650 to 950 degrees centigrade is preferred.

This invention may be carried out at atmospheric or at elevatedpressures as understood in the cracking art.

The addition of a nitrogen oxide into the hydrocarbon feed stream has nomaterial effect on the distribution of the cracked products. The productdistribution, after the hydrocarbons are cracked either with or withoutusing a nitrogen oxide product, are approximately the same.

The accompanying figure is illustrative of a method of carrying out thisinvention as described in the following example.

Example In order to demonstrate the effectiveness of the addition of anitrogen oxide to the feed stream in a hydrocarbon cracking process, thefollowing experiment was conducted.

Cracking apparatus was assembled as illustrated in the accompanyingfigure. The cracker 14, made of 1 inch S8446 pipe, was maintained at 830C. and the preheater 10 at approximately 400 C. Propylene (98.6 percentpure) was passed through the unit at 200 liters per hour. Nitric oxide(99 percent pure) was introduced through a Rotameter at 4.4 liters perhour. The cracking process was carried out for 3 hours. The liquidproducts were collected in the receiver while the gaseous ones wereanalyzed and their volume measured with a wettest meter.

The propylene feed stream was closed after the three hours and a 50-50air-argon mixture was then passed through the reactor at such a ratethat the heat of combustion of the carbon raised the tube temperature to1000 C. Burning of the carbon continued while the temperature wasmaintained at 1000 C. by increasing the amount of air into the reactorand at the same time, proportionately decreasing the amount of argonentering the reactor. Burn-out was complete at 6 minutes whence thetemperature of 1000" C. could no longer be maintained notwithstandingthe quantity of air allowed to enter the reactor.

For comparison, the same cracking procedure was performed, but Withoutthe addition of nitric oxide to the prolylene feed stream. After threehours, the propylene feed was terminated and the same burn-out procedureaforementioned was performed. Burn-out time required to relieve thereactor tubes of carbon clogged therein was 1 /2 hours.

Thus, it is seen that the addition of nitric oxide greatly decreased theamount of carbon formed as a solid coke during the cracking process.

An analysis and comparison of the product distribution resulting fromboth of the aforementioned runs revealed no appreciable differenceexcept for the presence of more oxide and nitrogen compounds in the runwherein nitric oxide was added to the feed stream.

Various modifications can be made in the method of the present inventionWithout departing from the spirit or scope thereof, for it is understoodthat I limit myself only as defined in the appended claims.

I claim:

1. In a process for the thermal cracking of hydrocarbons, theimprovement which comprises admixing from 0.1 to 10 mole percent of anoxide of nitrogen to the hydrocarbon feed prior to subjecting such feedto thermal cracking temperatures.

2. The process of claim 1 wherein the oxide of nitrogen is added to thehydrocarbon feed in an amount of from 5 0.5 to 3 mole percent.

3. The process of claim 1 wherein the oxide of nitrogen is nitric oxide.

References Cited by the Examiner UNITED STATES PATENTS 2,660,032 11/1953R osenthal 208129 2,178,365 4/1965 Miale 208114 DELBERT E. GANTZ,Primary Examiner.

H. LEVINE, Assistant Examiner.

1. IN A PROCESS FOR THE THERMAL CRACKING OF HYDROCARBONS, THEIMPROVEMENT WHICH COMPRISES ADMIXING FROM 0.1 TO 10 MOLE PERCENT OF ANOXIDE OF NITROGEN TO THE HYDROCARBON FEED PRIOR TO SUBJECTING SUCH FEEDTO THERMAL CRACKING TEMPERATURES.